Chapter 1.1, Problem 37ES

Given a function $f$ and a real number $a$ , explain informally why $\underset{x\to 0}{\lim }f\left(x+a\right)=\underset{x\to a}{\lim }f\left(x\right)$ (Here “equality� means that either both limits exist and are equal or that both limits fail to exist.)